1. Field
This technology relates generally to coal and/or mineral spirals and, more particularly, to splitter controls for coal spirals.
2. Background Art
Spiral concentrators have been utilized in the mineral industry for treatment of chrome-bearing sands since the 1940's. In the 1950's it was demonstrated that reasonable separation between low ash clean coal and high ash mineral matter could be achieved using coal spirals. Coal spirals are widely used in coal preparation plants around the world to clean fine coal, typically in the particle size range of 1×0.15 mm. Recent studies also report that high efficiency separation can be obtained for fine coal cleaning having a particle size as small as 45 microns. A spiral concentrator is a flowing film separator in which the lightest particles move to the outermost section of the spiral profile, whereas the heaviest particles remain in the inner most section. There are usually two splitters at the discharge end of a coal spiral to produce three product streams (i.e. clean coal product, middlings, and tailings respectively). The splitter position which decides the clean coal yield and product quality, is typically set at one point during the initial installation and is rarely moved again, and if it is moved it is performed manually. This results in a significant loss of clean coal to the tailings stream with fluctuating feed characteristics.
The major factors which have made spiral concentrators so popular include low capital investment and low operating costs and there are no requirements for chemical reagent or dense medium. Despite their popularity and the trend toward increased automation in modern coal preparation plants, adjustments to the controllable process variable for coal spirals, i.e., product splitter position, continue to be done (if at all) manually. Since spiral feed in a plant tends to fluctuate on a regular basis, due to the change in run-of-mine coal characteristics, suitable manual adjustment of splitter position in tens or hundreds of spirals operating in a plant is nearly impossible. As a result, the clean coal yield from a spiral and also the overall plant suffer on a regular basis. There can be a significant variation in spiral feed ash content and spiral feed solid content. These fluctuations in feed resulted in a significant change in spiral performance, which can be described by a variation in product ash content over the range of 7.75% to 12.95% and clean coal yield from 62.41% to 82.72% for a specific operating plant.
A comprehensive coal preparation plant of the modern day consists of four cleaning circuits, utilized to clean coal of different sizes which range from 100 mm (4 inch) to 0. If one were to name these circuits by the size of the coal they clean, these four circuits could be teemed as coarse, intermediate, fine and ultrafine coal cleaning circuits. It is the fine coal circuit that utilizes spiral concentrator as the coal cleaning technology in most of the plants. Nearly 6 to 7% of the coal produced worldwide go through the spiral cleaning process in coal processing plants.
Traditionally spiral concentrators separate clean coal from ash forming mineral rejects using the general principle of flowing film separation in the particle size range of 1×0.15 mm. The product/reject splitter is a key performance controller of a conventional coal spiral. The quantity and quality of clean coal produced from a spiral concentrator is directly dependent on the splitter position, i.e., how far the splitter is positioned from the central column on the spiral trough. Spiral splitters position is adjusted manually when a significant change in the feed coal characteristics (include, solid/liquid content, ash content, sulfur content, washability etc.) is expected to occur to continue producing the same incremental quality clean coal. However, an average size plant has to have a lot of these spirals to clean the entire fine coal since spiral is a low capacity processing unit. Also, because of the large relative foot print they take, spiral banks in a plant are typically very tightly packed with the individual spirals. Thus, the manual adjustment of spiral splitters, although physically possible, is rarely ever done after the initial installation of the plant. This leads to loss of clean coal which could have been recovered with the due adjustment of the splitter position in a timely manner.